In-plane charge fluctuations in bismuth-sulfide superconductors

The local atomic structure of the new nonmagnetic superconducting system LaO_1-xF_xBiS₂ is investigated using neutron diffraction and the pair density function analysis. Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting LaO_1-xF_xBiS₂ is presented. In-plane short-range distortions of sulfur atoms up to 0.3 Å in magnitude break site symmetry and create two distinct environments around Bi. Out-of-plane motion of apical sulfur brings it closer to the La-O/F doping layer with increasing x that may lead to a charge transfer conduit between the doping layers and the superconducting BiS₂ planes. The mechanism for superconductivity may arise from the interplay between charge density fluctuations and an enhanced spin-orbit coupling suggested theoretically that induces spin polarization.